Drug delivery devices have become widespread in the field of medical treatment where regular injection of a medicament by persons without formal medical training occurs. For example, this may be increasingly common among patients having diabetes where self-treatment enables such patients to conduct effective management of their disease.
There are different types of drug delivery devices. On the one hand, one can differentiate between resettable devices which are reusable and non-resettable devices which are disposable. For example, the disposable delivery devices are supplied as self-contained devices. Such self-contained devices do not have removable pre-filled cartridges. Rather, the pre-filled cartridges may not be removed and replaced from these devices without destroying the device itself.
On the other hand, one can differentiate between so-called fixed dose devices which only allow dispensing of a predefined dose without the possibility to increase or decrease the set dose and so-called variable dose devices which allow a user to individually select and dispense a number of user variable doses of a medicament. In general, all these types of delivery devices can be pen-shaped.
The drug delivery devices as explained above generally comprise several primary elements, for example a cartridge section that includes a cartridge which can be contained within a housing or holder, a needle assembly connected to one end of the cartridge section, and an additional assembly which may enable dialing and/or delivery of a dose of a medicament. The latter assembly can be connected to the end of the cartridge section which is located away from the needle assembly. A cartridge (often referred to as an ampoule) typically includes a reservoir that is filled with a medication (for example insulin), a movable bung or piston or stopper located at one end of the cartridge reservoir, and a top having a pierceable seal or sheath located at the other, often necked-down end.
The assembly for dialing and/or delivering a dose of a medicament may comprise a housing part and a piston rod or lead screw which is displaceable relative to the housing part. The piston rod is designed to push the bung or stopper of the cartridge in a direction to an outlet of the cartridge such that the medication can be expelled out of the device and delivered to a patient. In an initial state, for example a supplied state of the device, there can be a certain gap or clearance between the end of the piston rod and the bung of the cartridge when these parts are assembled. For example, the mentioned gap can be a consequence of tolerances associated with all or many of the assembled parts and/or the desire not to pre-load the bung of the cartridge in the assembled state of the device. The latter fact would have the consequence that medication might be unintentionally expelled out of the device.
A gap between the end of the piston rod and the cartridge bung can have several disadvantages. One disadvantage may arise if a user takes the device for the first time for an intended injection of a predetermined dose of a medicament. The actual dose received will then be equal to the predetermined dose less the initial air gap between the piston rod and the bung. For example, this air gap is typically equivalent to a loss of between 0 and 0.14 ml on the first dose. For many drugs, this discrepancy can be significant and is well outside the allowable dose accuracy limits. At present, it is conventional use, therefore, for the user to be instructed to perform a so-called “air shot” which means that the user has to operate the device for the first time without injecting the medication into his body but in the air until the gap between the piston rod and the bung is traversed and fluid may begin to be ejected.
This course of action has the additional disadvantage that expensive medication potentially has to be discarded before the user may take the first injection of the medicament. This may result in rising costs for the medication and/or the drug delivery device.
It is, therefore, an object to describe an assembly for a drug delivery device as well as a drug delivery device with such an assembly which are designed so that the gap between the piston rod and the cartridge bung can be removed (transition from an initial state to a primed state) before a first dose of a medicament is injected.